A variety of techniques for controlling the transverse position of a magnetic recording/playback head relative to a magnetic tape have been proposed and used in the past. For example, simple mechanical detents have been employed for positioning the heads at a series of transverse positions relative to a tape transport path defined by tape guide members. Such a technique may be suitable for systems such as those designed for use with eight track audio cartridges in which only eight tracks are recorded on one-quarter inch wide tapes. In most current systems, the recording tape is contained within some type of cartridge or cassette which also includes tape guide members. These members, either solely, or in combination with other members forming a part of the tape drive, control the transverse position of the tape such that any variation in the positioning of the cartridge within the drive or in the tolerances of the cartridges themselves will result in a variation in the final transverse position of the tape. Such a final position will vary even as the same cartridge is repeatedly removed and reinserted in the same drive. Such variations can be tolerated in systems in which each track is relatively wide and is separated by a similarly wide space from an adjacent track, such as in the eight track audio cartridge system.
Newer systems, in which significantly narrower tracks are desirably used and in which the tracks are placed much closer together, such as when over twenty tracks are to be recorded on 6.4 mm wide tape, have been found to require a much tighter control over the transverse position of the heads relative to the tape. Accordingly, methods have been developed for sensing the actual tape edge, apart from the specific positioning of a cartridge or cassette within which the tape is located. Thus, for example, optical and mechanical sensors have been so employed, with such additional components contributing to the complexity and expense of the drive and providing additional sources of malfunction.
It has also been previously proposed to detect the edge of the tape magnetically. Thus as disclosed in U.S. Pat. No. 4,422,111 (Moeller and Wolff), a magnetic tape may be pre-recorded with patterns which extend from one edge of the tape to the other, and a playback head thereafter incrementally positioned at successive positions ever closer to an edge of the tape. The prerecorded patterns are thus sensed until the head moves off the edge, at which point no further playback signal is detected, thus establishing a head reference position corresponding to the edge of the tape, and from which a plurality of parallel tracks along the tape may be indexed. The system there disclosed was adapted only for use with such a preformatted tape.
U.S. Pat. No. 4,476,503 (Solhjell) also depicts a method for magnetically sensing the edge of tape via recorded patterns bordered by one edge of the tape. As there shown, a combined read after write head is mounted on a single support. As the tape is moved longitudinally, the support is moved transversely toward the tape from a position off the edge of the tape. At the same time, a signa1 is applied to the write head and any thus recorded signal is immediately reproduced by the read head. The edge of tape is proposed to be determined by comparing the read signal to a constant prescribed reference value, or by determining the difference between two read signals obtained at different points in time and comparing that difference to a prescribed reference value. While the techniques depicted in these patents may be useful in certain circumstances, each has certain limitations: the method of Moeller et al. can only be used with preformatted media, while that of Solhjell suggests only the use of a fixed reference value.